Method of providing an inert atmosphere



9-456 XR 393899829 SR1 5;);

X pm June 25, 1988 A. E. STANFORD 3,389,829 I I} METHOD OF PRQVIDING AN man ATMOSPHER? v X f Original Filed April 24, 1963 q 2 Sheets sh t 1 FIGURE l ALFRED E. STANFORD INVENTOR PATENT ATTORNEY v V A. E. STANFORD METHOD OF PROVIDING AN INERT ATMOSPHERE June 25, 1968 Sheets-Sheet Original Filed April 24 1963 N mmamc 52 now Q0. N9

ALFRED E. STANFORD INVENTOR BY v PATENT ATTORNEY 3,389,829 METHOD OF PROVIDlYG AN INERT ATMOSPHERE Alfred E. Stanford, Oakland, N.J., assignor to Esso Research and Engineering Company, a corporation of Delaware Original application Apr. 24, 1963, Ser. No. 275,249, now Patent No. 3,285,711, dated Nov. 15, 1966. Divided and this application Mar. 7, 1956, Ser. No. 532,295

' 4 Claims. (Cl. 220-88) ABSTRACT OF THE DISCLOSURE A method of cooling" and treating hot flue gases to remove oxides of sulfur therefrom and lower the relative humidity of the gases. The gases are first scrubbed by contact with an alkaline solution of sea water and subsequent-1y cooled by contact with additional refrigerated sea water. Subsequently the flue gases thus dehumidified are raised in temperature by a heater prior to passage into the hold of a cargo tanker to introduce therein an inert non-combustible gas as combustible liquid is withdrawn from the hold. Provision is made for recirculating a portion of the scrubbed dehumidified gases through the in the cargo hold.

This application is a divisional application of Ser. No. 275,249, filed Apr. 24, 1963, now US. Patent No. 3,- 285,711.

The present invention concerns an improved flue gas inerting system. In particular, it relates to a system which utilizes flue gas derived from boilers as an inerting atmosphere for filling cargo holds in tankers in order to prevent an explosive atmosphere from forming. It further relates to an improved system for producing truly inert flue gases in that such gases will not be corrosive t the tank walls and pipes in the tankers holds.

The cargo tanks in oil tankers normally contain hy drocarbon vapors in the space above a loaded cargo. Also, during a ballast voyage, enough residual cargo usually remains (unless special cleaning has been provided) to saturate the vapor space with hydrocarbons. A combustible vapor, gas or mist will not burn or explode, however, if the oxygen content of the atmosphere in the hold is reduced-below a certain definite value, which varies with the combustible material under consideration. It is known that the most critical point from the danger viewpoint, occurs when the tanker is beingunloaded. As the cargo is pumped out, it is necessary to replace the lost liquid volume with gas in order to maintain pressure balance. If an air vent is used, the atmosphere in the hold will contain a greater than critical concentration of ox gen and any flame or spark could set offthe entire cargo. It is therefore imperative that this venting gas be one that will yield anatmosphere having a lower than 3,389,829 Patented June 25, 1968 inert gases. Since the main objective of these pseudo-inert gases is to provide an oxygen poor atmosphere, an excel lent source of these gases is the flue gases produced by the combustion of fuel'oil in the boilers.

An example of the composition of flue gas produced by the combustion of No. 6 oil burned in a Cleaver- Brooks burner, Model CB-50 is given below:

PRODUCTS OF CUMBUSTION OF NO. 6 FUEL OIL l il'lhe oil contained 2.53% sulfur and 385 ppm. vanadium with 0.09% as Y The C0; in the line gas was 13.9%no CO was found at a stack temperature of 320 F.the 02 concentration is approximately 3%.

9 1,490 p.p.m.

It is obvious from an examination of the above products that such a flue gas is far from inert in fact. Both sulfur gases are extremely acid and are very corrosive to. metal when brought in contact with it over a period of time. Furthermore, the combustion of fuel oil yields substantial amounts of carbonaceous particulate matter which of necessity must be removed before the flue gas is allowed toenter the cargo holds. If traditional methods of scrubbing gases are used to remove the above contaminants, large quantities of water will be carried in the 'resulting gas stream. This water, then, will become the main corrosion causing agent and will therefore greatly reduce the effective use-life of the cargo holds.

It is therefore an object of this invention to prevent flame and explosions in the cargo holds of tankers by providing therein a non-combustion supporting atmosphere. It is a furtherobject of this invention to provide a truly inert atmosphere which will be non-corrosive to the cargo holds.

The system of the present invention produces an inert flue gas by the following steps. Flue gas, produced in the boiler of the tanker or other cargo vessel, is led from the. boiler uptakes to a scrubber which preferably uses salt water. The scrubber serves to remove virtually all of the soot and other particulate matter. Additionally, the scrubber cools the flue gas from its original temperature of about 350 F. to about 80 F. Of course it would be possible to utilize a fresh water scrubber 'here also, but salt Water is far more convenient and is preferable for that reason.

. The acid sulfur gases are then removed from the flue gas stream by passing the gas through an alkali scrubber.

the scrubber contain Na- CO for the purposes of the present invention.

critical level of oxygen. In most cases, if the oxygen concentration of the atmosphere in the hold is kept below approximately 12%, flames will not propagate nor will explosions take place, regardless of the concentration of the combustible that may be present. As examples, the minimum oxygen content for gasoline to burn is 14.4%, while for benzene, it is 13.9%.

One possible solution would be to use the classic inert gas atmospheres to vent the cargo holds during unloading. Gases such as helium, nitrogen, carbon dioxide, argon, etc., are known for their inability to support combustion. However, such inert gas systems would be far too expensive to be used in commercial tankers having gargantuan hold volumes. Therefore, pseudo-inert gases have been utilized in lace of the more expensive true It is within the scope of this invention to utilize a combin'ed caustic-salt water scrubbing step to simultaneously effect the particulate and acid gas removal and temperature reduction. In such an embodiment the caustic agent is added directly to the salt water stream prior to its circulation through the scrubber.

Gas, freed from particulate matter and acid gas, is then chilled by passing it through a dehumidifying coil. The flue gas is cooled to about 45 F. and at this temperature most of the water in the gas stream is condensed out. The cooling element may be a type employing Freon or similar halogenated paraffinic gases, chilled water, or any other refrigerantknown. to the art, as coolant, and can be used in various ways. It can be used to cool the gases directly by heat exchange through a cooling coil. Another possibility is to-use the cooling coil to chillwvater shunted *thetransfer of momentum from-gas jets to'the liquid flowoff from the main scrubbers intake and then use this water to wash the flue gases arising from the main scrubber. .Other variations will suggest themselves to one experienced in the refrigerant art.

The cooled, dehumidifled gas is then passed through a-heater in which the gas temperature is raised to about 110 F. in order to prevent condensation in the piping as the gas is passed to the cargo holds. A preferable form of heater is a steam air heater since the steam used for its operation is readily available on marine vessels.

The flue gas inerting system of the present invention will be more readily understood by reference to the appended drawings and the description thereof.

FIGURE 1 is a diagrammatic representation of a typi cal flue gas inerting system in a marine cargo vessel.

FIGURE 2 is a diagrammatic view of a specific embodimeat of an'inert gas system of the present invention.

Turning now to FIGURE 1, a general view of a flue gas inerting system is shown in relation to other facilities in a this flue gas tap system is reproduced identically by flue 14, outlet line 15, butterfly valve 16 and line 17. The gas flows are combined in the latter case into a single line 18 which serves as an inlet for the scrubber unit 19. Salt water for the scrubber is taken aboard vi-a line 20 by the suction action. of pump 21. The salt water is then fed into scrubber 19 by line 22. The salt water rate of flow is controlled by valve 23. Salt water discharge from scrubber 19 is returned to the sea by means of outlet line 24. A control valve regulates the outlet flow.

The dehum-idified inert flue gas emerging from the scrubber unit passes through gas outlet line 26. A safety cutout valve 27 is located in this line to prevent excessive pressure ditferentials from building up. Blower 28, powered by steam turbine 29, forces the gas into gas supply header 30. Control of gas flow in the header is maintained by means of swing check valve 31 and butterfly isolation valve 32. Inert flue gas is supplied to individual cargo holds 34 by bleed off branch lines such as line 33 which taps off gas from supply header 30. Each branch line is equipped with a butterfly valve such as valve 35 to allow control over the course and amount of gas flow to the holds. The gas is bled directly into each hold through a tank head 36.

Turning now to FIGURE 2, a detailed diagrammatic view of the inert gas system is shown in a specific embodiment. Flue gas from the boilers 100a and 100!) is bled from flue 100a by line 101. The flow of flue gas through this line is controlled by butterfly valve 102. The gas then passes into the scrubber unit 103 by way of gas inlet line 104. a

The first element in the scrubber unit is a sea water scrubber 105. The flue gas vapors entering the scrubber are intimately contacted with a countercurrent stream of sea water. A suitable scrubber for this purpose utilizes a plurality of impingement balfle stages. The perforated plate in each impingement baffle stage consists of a sheet having about 600 holes per square foot, giving an open area of about 22 percent. Each set of impingement baflles is arranged so as to have a battle located directly above each perforation. Flue gas passes through the holes in the perforated plate and impinges on the battles. The relatively high velocity, in the order of 75-100 feet per second, required by the aerosol in the perforations is effective in atomizing the sea water at the edges of the perforations. The resultant mist particles carry into the vapor space above the baffle stage and continue to act as effective targets for finedust. It is probable that for small particles, most of the scrubbing action of the impingement baflle plate may be attributed to the spray droplets formed by ing over the perforated plate, rather than to the impingement of the particles on thebaflles.

Another desirable type of scrubber for use in element 105 is one which utilizes'a plurality of trays of bubble caps to contact flue gas countercurrently in once-through operation. The bubble caps are the familiar round bell caps with slots spaced equally around their lower periphery. The total slot area for each cap is about 12 square inches and there' are about49 caps per tray. Flue gas contacts the sea water in passing through the slots in the bubble caps. At the maximum flue gas flow rate for this type unit, about 4,300 s.c.f.m., the slot velocity is near 18 feet per second. Proportionately lower slot velocities would occur for lower flue gas flowrates. In general the range of flue gas flowrates would be about 2,000 to 4,300

s.c.f.m.

The cooling eflficiency of both the above described countercurrent scrubber elements was tested in use aboard tankers. A measure of cooling efliciency for the scrubbing operation is the so-called approach temperature. This temperature is thediiference between the flue gas temperthe flue gas from the inlet to the outlet. These values are summarized for each type of scrubber element in Table 1.

TABLE 1.-COOLINGS EFFICIENCY OF SEA WATER CR UBBE RS Approach Differential Temperature Flue Gas Temperature Difierential Scrubber Element Flow Rate, Between Inlet Between Out- I s.c.f.m. and Outlet; let Flue Gas Flue Gas, F. and Inlet Sea Water, F.

1. Implngerneut Bathe 1 2,000 278 3-5 (at 250 g.p.m. sea 2 2, 300 344 4-8 water). 1 2, 600 276 3-6 l 1 3, 200 270 4-5 2. Bubble Caps (at 200 l 700 302 17-18 g.p.m. sea water). 1 2, 700 301 12-1-t l 4, 300 301 12-14 1 Sea water scrubber element only. 2 Sea water scrubber element and a separate NazCO; absorber.

Theirnpingement baflie scrubber, using 250 g.p.m. of sea water, has an approach temperature which appears to hold fairly constant over the range of flue gas flow encompassing 2000-3000 s.c.f.m. (38 F., with an average of about 5 F.). The bubble cap scrubber, using 200 g.p.m. sea water shows an approach temperature of about 13 F. over the range 2700-4300 s.c.f.m. The temperature differential increases to about 1718 F. when a low flow of about 700 s.c.f.m. In general, the amount of sea water used in either type of scrubber will be in the range of about 5 to 280 gallons/1000s.c.f. flue gas. A preferable range is about -125 gallons/ 1000 s.c.f. flue gas.

' Sea waterfor scrubber unit 103 is obtained through line 106 and is regulated by control valve 107. Since the inert-. ing system will be used most extensively for unloading operations on'the vessel, it would be expected that this will occur mainly while the vessel is in harbor. Since most harbors have a great deal of suspended matter in the water and since one of the objects of the scrubber system is to being that of particulate scrubber and the second being that of S0 absorber.

The particulate-free, SOg'fl'CC flue gas then rises through scrubber unit 103 to the dehumidification zone 111. At this point the gas is at agemperature of about F. and

- ning the diverted sea water portion through refrigeration unit 114. Coolant for this refrigeration unit may either be a Freon type gas which is cycled through appropriate compression equipment (not shown) or else it may take the form of cold water supplied from the vessels air con ditioning system if an appropriate steam-jet system is already used for that purpose. As previously indicated, the inert gas system would be used most extensively when unloading the vessel. Since this, in the case of most crude products, will take place in harbors in the temperature zone, the loss of some or all air conditioning efiiciency during the unloading period will not impose a very great burden on the crews comfort.

The chilled sea water will descend countercurrently to the rising flue gas vapors in unit 103. As the water descends, it will be warmed to about the temperature found in scrubber element 105- and will then join the sea water inlet flow coming from line 110. The combined sea water streams are discharged through line 115 to the sea. It is possible, however, to recycle at least some of the exhaust stream by means of line 115a to the Na CO enriched stream thereby cutting down on the amount of Na CO needed to'maintain an efiicient absorbing concentration.

After passing through the chilled sea water, the flue gas is at a temperature of about 45 F. and is thereby effectively dehumidified since very little moisture vapor will be retained by the gas at that temperature. The inert flue gas then passes through heater 116, which preferably utilizes steam coils, wherein the gas temperature is raised to about 110 F. The war-med gas exits unit 103 by means of line 117. An automatic cut-out valve 118 controls the flow rate of the gas to the cargo holds. Distribution of the gas is effected by passing it through steam turbine blower 119. The distribution fiow rate is controlled by check valve 120, and butterfly valves 121 and 122. Cargo hold 123 receives the flue gas flow through inlet head 124.

In order to guarantee the best particular and S removal efficiency of the scrubber, a recirculation line 133 is incorporated from the discharge of the blower 119 to the suction piping of the scrubber104. A modulated butterfly valve 134 is installed in line 133 to control the amount of recirculation. The purpose of this connection is to guarantee that the design gas flow rate and velocities are maintained constant through the scrubber trays no matter what the cargo discharge rate is from the tanks.

In addition, the system is provided with an alternate piping arrangement to permit isolation of the scrubber components and the taking of suction for' an atmospheric intake for purposes of gas freeing. Gas freeing air is drawn in through air intake 125. Blind 126 is opened thereby allowing air to enter scrubber unit 103. The air passes up into line 117 and is then diverted into the cargo piping system 129 by opening blind 127 (which is normally closed during the inerting process) and closing butterfly valve 121. Cargo tank 123 can then be blown free of the inert gas atmosphere by the opening of cargo tank suction valve 128. Exhaust gas is released to the atmosphere by closing valve 122 and opening the tank hatch 132.

The system as described above will yield a truly inert flue gas. The concentration of S0 in this gas is in the range of 0.5-5 ppm. as opposed to a concentration of about 3000 p.p.m. in the untreated flue gas. Further-more, water vapor content is about 0.80% as contrasted to the 9.68% originally found in the flue gas.

In order to more dramatically prove the effectiveness of the present system, a comparative corrosion rate test was run utilizing a sea water scrubber of'the impingement bafiie type identified as Unit A against a scrubber system of the present invention also using an impingement bathe type element identified as Unit B. The tests were conducted on corrosion test racks exposed to the scrubbed flue gas from each of the flue gas systems for six months. The results of the tests are summarized in Table 2.

, TABLE 2.-CORROSION RATES FROM TEST RACKS Corrosion Rate, m.p.y. Material Unit A Unit B Carbon Steel:

Average 18 I 2.4 N odular Cast. Iron:

Average 11. 5 2. 9

It is therefore apparent that flue I gas produced by the I system of the present invention is very much less, corrosive than even scrubbed fiue gas. Therefore, the use of such a system will substantially increase the useful life of all components in the vessel contacted by the flue gas.

While several specific embodiments of the instant invention have been recited with some particularity, it.

should be emphasized that these embodiments should not be considered as being limiting of the scope of the invention. It is well within the province of an engineer skilled in the art to modify the system in many ways without departing from the spirit of the invention.

What is claimed is:

1. The method of providing an inert atmosphere for a cargo hold at ambient-temperature comprising the steps of producing flue gases having an elevated temperature and treating said flue gases to remove oxides of sulfur' cool and remove oxides of sulfur from said gases, cool-- ing additional sea water to a temperature lower thanthe cooled gases, placing the additional sea water into intimate direct contact with said gases to cool and therebydehumidify said gases, thereafter raising the temperature of said dehumidified gases tolower the relative humidity thereof, and directing said flue gases to the cargo hold.

2. The method of claim 1 including the further step of recirculating a portion of the treated flue gases to be retreated as a'function of the demand for flue gases in said cargo hold to thereby maintain a substantially constant gas fiow rate during said contacting, cooling and heating steps irrespective of what the cargo discharge rate is from the cargo hold.

3. The method of claim 1 wherein the dehumidified gases are heated to about F. prior to being directed to said cargo hold.

4. The method of claim 1 wherein the cooled additional sea water cools the flue gases to about 45 F.

References Cited RAPHAEL SCHWARTZ, Primary Examiner. 

